The present invention relates to a charging device for charging a member to be charged in contact with the member to be charged, an image forming apparatus, such as a copying machine and a printer, and a process cartridge detachably mountable to the image forming apparatus. The present invention also relates to a charging roller included in the charging device.
Hitherto, in image forming apparatus such as an electrophotographic apparatus and an electrostatic recording apparatus, a corona charger (corona discharger) has been frequently used as a charging device for uniformly charging an image bearing member (member to be charged) such an electrophotographic photosensitive member and an electrostatic recording dielectric member to a prescribed potential of a prescribed polarity. The corona charger is a non-contact type charging device and typically has an organization including a discharge electrode, such as a wire electrode, and a shield electrode disposed so as to surround the discharge electrode except for a discharge opening. The discharge opening is directed toward and free of contact with an image bearing member as a member to be charged, whereby an image bearing surface of the image bearing member is charged by exposure to a discharge current (corona shower) caused by applying a high voltage between the discharge electrode and the shield electrode.
In recent years, in view of advantages such as lower ozone generation and lower power consumption compared with such a corona charger, a contact-type charging device (contact charging device) as mentioned above, wherein a charging member supplied with a voltage abuts against a member to be charged to charge the member to be charged, has been commercialized.
More specifically, such a contact charging device includes an electroconductive charging member of a roller-type (charging roller), a fur brush-type, a blade-type, etc., which is caused to contact a member to be charged, such as an image bearing member, and is supplied with a prescribed charging bias voltage to charge a surface of the member to be charged to a prescribed potential of a prescribed polarity.
The charging mechanism in the contact charging includes two types of mechanisms, i.e., (1) a discharge charging mechanism and (2) a direct injection charging mechanism, in mixture, and various charging characteristics appear depending on which of the two mechanisms is dominant. FIG. 4 illustrates charging characteristic curves according to the representative of the respective mechanisms, which are characterized as follows.
(1) Discharge Charging Mechanism
According to this mechanism, a surface of the member to be charged is charged by a discharge phenomenon occurring at a minute gap between the contact charging member and the member to be charged. As the discharge phenomenon occurs above a certain discharge threshold voltage between the contact charging member and the member to be charged, it is necessary to supply the contact charging member with a voltage larger than a charge potential provided to the member to be charged. Further, according to this mechanism, the occurrence of discharge by-products is inevitable in principle while the amount thereof is remarkably less than in the case of a corona charger, so that difficulties such as the occurrence of ions caused by active ions can not be completely obviated.
For example, while the roller charging scheme, using a conductive charging roller as a contact charging member, is preferred in view of its charging stability and is widely used, the discharge charging mechanism is generally predominant in the roller charging scheme.
More specifically, a charging roller is generally formed of a conductive or medium-resistivity rubber material or foam product, which may be laminated with another layer to provide desired properties. The charging roller is provided with an elasticity so as to form a certain width of contact (nip), which results in a large friction during its movement following the movement of the member to be charged at an identical speed or with a slight speed difference. During this contactive movement, some non-contact state is inevitably caused due to a surface shape-irregularity of the roller and material attached to the member to be charged, which promotes the discharge charging mechanism. Thus, as the charging is effected by discharge from the charging member onto the member to be charged, the charging is started by application of a voltage in excess of a certain threshold voltage. For example, in a case where a 25 xcexcm-thick OPC photosensitive member (photosensitive layer) as a member to be charged is charged by means of a charging roller abutted thereto, the surface potential of the photosensitive member starts to increase when a voltage of approximately 640 volts or higher is applied to the charging roller, and is increased thereafter linearly in proportion (at a rate of 1) to the increase in the applied voltage (2) Direct injection charging mechanism.
According to this mechanism, a surface of a member to be charged is charged with charges directly injected from the contact charging member to the member to be charged as proposed in Japanese Laid-Open Patent Application (JP-A) 6-3921 and JP-A 11-65231.
In the direct injection charging mechanism, charges are directly injected to the surface of the member to be charged by the direct contact of a medium-resistivity contact charging member to the surface of the member to be charged basically without relying on the discharge mechanism. Accordingly, the member to be charged can be charged to a potential proportional to the voltage applied to the charging member even at an applied voltage below the discharge threshold votlage as represented by a solid line in FIG. 4. The direct injection charging mechanism is not accompanied by the occurence of ions and is therefore free from the difficulties of discharge byproducts.
More specifically, a contact charging member, such as a charging roller, a charging brush or a charging magnetic brush, is supplied with a voltage to inject charges to charge-retaining means, such as a trap level present at the surface of the member to be charged (image-bearing member) or electroconductive particles in a charge-injection layer formed at the surface of the member to be charged. As the discharge phenomenon is not preodiminant, the necessary voltage applied to the charging member is only a desired potential to be provided to the member to be charged, so that the occurrence of ozone can be obviated. The use of a porus roller, such as a sponge roller, coated with electroconductive particles for promoting the contact charging performance as a contact charging member, is known to be effective for enhancing a dense contact between the contact charging member and the member to be charged. surface of the member to be charged. As the discharge phenomenon is not predominant, a necessary voltage applied to the charging member is only a desired potential to be provided to the member to be charged, so that the occurrence of ozone can be obviated. The use of a porous roller, such as a sponge roller, coated with electroconductive particles for promoting the contact charging performance as a contact charging member, is known to be effective for enhancing a dense contact between the contact charging member and the member to be charged.
However, it is not an easy matter to sufficiently charge a member to be charged by using a simple device structure including a charging roller, so that charging irregularity is liable to be caused due to local charging failure. The charging irregularity leads to image irregularity in the resultant images. The charging irregularity may be ascribed to failure in effective retention of electroconductive particles coating the sponge roller for enhancing the injection charging because the sponge cell is provided with sponge cells formed by gas generation caused by decomposition of a foaming agent in a larger size and in a lower density than expected.
Accordingly, a principal object of the present invention is to provide a contact charging device of the type wherein a member to be charged is charged by a charging member with the aid of electroconductive particles disposed between the charging member and the member to be charged, allowing uniform direct injection charging.
Another object of the present invention is to provide an image forming apparatus including such a charging device.
A further object of the present invention is to provide a charging roller suitably included as a charging member in such a charging device.
According to the present invention, there is provided a charging device, comprising: a member to be charged, a charging member disposed so as to form a nip with the member to be charged, and electroconductive particles disposed so as to be present at the nip between the charging member and the member to be charged, wherein the charging member has a fine network-textured surface provided with concave cells giving a total cell edge perimeter per unit area of 15 mm/mm2 to 60 mm/mm2.
According to the present invention, there is further provided an image forming apparatus, comprising: a photosensitive member, a charging member disposed so as to form a nip with the photosensitive member, and electroconductive particles disposed so as to be present at the nip between the charging member and the photosensitive member, wherein the charging member has a fine network-textured surface provided with concave cells giving a total cell edge perimeter per unit area of 15 mm/mm2 to 60 mm/mm2.
The present invention further provides a process cartridge, comprising: a photosensitive member, a developing means and a charging means integrally supported to form a cartridge which is detachably mountable to a main assembly of an image forming apparatus, wherein
the charging means includes a charging member disposed so as to form a nip with the photosensitive member, and electroconductive particles disposed so as to be present at the nip between the charging member and the member to be charged, wherein the charging member has a fine network-textured surface provided with concave cells giving a total cell edge perimeter per unit area of 15 mm/mm2 to 60 mm/mm2.
The present invention also provides a charging roller, having a fine network-textured surface provided with concave cells giving a total cell edge perimeter per unit area of 15 mm/mm2 to 60 mm/mm2.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.